Resins for extrusion coating on paper, board, aluminum etc, are designed with broad MWD (molecular weight distribution) and low extractables. In extrusion coating application the polymer is processed at high temperature conditions, typically above 280° C. and below 350° C. Broad MWD (high molecular weight fraction) is necessary for good processability during coating (neck-in and draw down balance), while low extractables are needed for low smoke formation during coating, at high temperature conditions, and or for food contact compliance. Broad MWD LDPE (low density polyethylene) is made up of low and high molecular weight polymer molecules, and an average molecular weight will determine the melt index. The extractable fraction increases with an increasing fraction of low molecular weight molecules, and is enhanced by increasing short chain branching frequency at low molecular weight molecules. In view of this combination of features, there is typically a trade-off between coating performance and extractable level.
Typically LDPE resins with broad MWD are made on autoclave or combination of autoclave and tube reactors. Broad MWD resins can be achieved in autoclave reactor systems by promoting long chain branching and through the inherent residence time distribution by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths.
The autoclave and tubular reactor systems differ from each other in respect to residence time distribution, typically uniform for tubular and dispersed for autoclave reactor zones, while polymerization conditions like temperature, pressure and polymer concentrations vary widely in tubular reactor systems and are uniform or are less differentiated for autoclave reactor systems. The uniform residence time in tubular reactor conditions leads to narrower MWD, therefore broad MWD can only be achieved in tubular reactors by applying extremely differentiated polymerization conditions. These extremely differentiated polymerization conditions lead to higher extractable level by formation of polymer molecules with lower molecular weight and/or increased short chain branching level in the low molecular weight fraction. However, an autoclave process typically operates at lower conversion levels, and is more capital/energy intensive than a tubular process.
Thus, there is a need for new ethylene-based polymers with broad MWD and low extractables, suitable for extrusion coating application, and which can be made in a tubular process. There is a further need for such polymers that can be prepared without any chemical modification, for instance the use of cross-linking agents in reactors, separators, extruders, etc., or the use of blending operations.
International Publication No. WO 2007/110127 discloses an extrusion coating composition comprising an ethylene copolymer. The ethylene copolymer is obtained by a polymerization that takes place in a tubular reactor at a peak temperature between 300° C. and 350° C. The comonomer is a bifunctional α,ω-alkadiene, which is capable of acting as a crosslinking agent.
International Publication No. WO 2006/094723 discloses a process for the preparation of a copolymer of ethylene and a monomer copolymerizable therewith. The polymerization takes place in a tubular reactor at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth) acrylate, and the comonomer is used in an amount between 0.008 mol % and 0.200 mol %, relative to the amount of ethylene copolymer. The di- or higher functional (meth)acrylate is capable of acting as a crosslinking agent.
European Patent EP 0928797B1 discloses an ethylene homo or copolymer having a density of between 0.923 and 0.935 g/cc, and a molecular weight distribution Mw/Mn between 3 and 10, and comprising from 0.10 to 0.50 wt % of units derived from a carbonyl group containing compound, based on the total weight of the homopolymer or copolymer.
DD276598A3 (English Translation) discloses a process for adjusting and regulating the input gas streams for multizone tubular reactors, with at least two side input streams, for the production of ethylene polymers, by free-radical bulk polymerization. The polymerization takes place at pressures above 80 MPa, temperatures from 373 to 623K, and in the presence of 10 to 50 ppm of oxygen, as polymerization initiator.
U.S. Pat. No. 3,334,081 discloses a continuous process for the production of polymers of ethylene as carried out in a tubular reactor, whereby the polymer is obtained at a higher conversion rate. In one embodiment, this patent discloses a continuous process for the polymerization of ethylene in a tubular reactor at a pressure of at least about 15,000 p.s.i.g., and a temperature from about 90° C. to about 350° C., in the presence of a free radical initiator.
U.S. Pat. No. 3,657,212 discloses a production of ethylene homopolymers having a specific density, by polymerization of ethylene, under the action of organic peroxides and oxygen as free-radical-generating polymerization initiators, and of polymerization modifiers, at elevated temperature and superatmospheric pressure, in a tubular reactor having two successive reaction zones. A mixture of ethylene, polymerization initiator, and polymerization modifier are introduced continuously at the beginning of each reaction zone. The ethylene homopolymers have a broad molecular weight distribution, and are said to be practically devoid of very high molecular weight constituents.
DD120200 (English Translation) discloses a process for producing homopolymers of ethylene with a bulk density of 0.912 to 0.922 g/cc, in tubular reactors, by polymerization of ethylene with free-radical forming initiators. The polymerization is carried out in the absence of chain regulators, and at maximum reactor temperatures of 250 to 340° C., and at pressures of 1000 to 2000 atm, and with multifunctional peroxides.
CA2541180 discloses polymer blends composed of from 25 to 75 wt % homopolymer produced in a tubular reactor, and 75 to 25 wt % of ethylene homopolymer in a high pressure autoclave reactor, provided that each homopolymer is removed from the reaction zone prior to being blended together. The blends so formed are said to have a good combination of neck-in and adhesion properties.
Two-zone tubular reactor systems, commonly used in the above art, lead to polymers with either too narrow MWD or too high extractable level (also see LDPE 160C in Table 4, which is also produced in two-zoned tubular reactor). Achieving broad MWD resins with these reactor systems typically require extremely high peak temperatures and/or low reactor inlet pressures, leading to formation of lower molecular weight material with increased short chain branching level, which leads to high extractables.
Additional polymerizations and/or resins are described in the following: U.S. Pat. Nos. 2,153,553; 2,897,183; 2,396,791; 3,917,577; 4,287,262; 6,569,962; 6,844,408; 6,949,611; U.S. Publication Nos. 2007/0225445; 2003/0114607; US2009/0234082; International Publication Nos. WO 2012/044504; WO 2011/075465; WO 2008/112373; WO 2006/096504; WO 2007/110127; GB1101763; GB1196183; DE2107945 (Abstract); EP0069806A1; EP1777238B1; EP0792318B1; EP2123707A1; and J. Bosch, “The Introduction of Tubular LDPE to the Extrusion Coating Market and the Specifics of the Product,” 12th TAPPI European PLACE conference, 2009, 1-20.
Conventional tubular polymerization processes of the art typically produce broad MWD polymers with high levels of extractables. Thus, there remains a need for new ethylene-based polymers, such as LDPE resins, with broad MWD and low extractables. These needs and others have been met by the following invention.